It is becoming increasingly common for herbicides to be applied in the form of oil-based emulsions. Such herbicides are highly efficient and very small quantities, if properly applied, can be used to treat large areas. However, to be effective, the herbicides must be applied in the form of droplets of uniform size and distribution. The nature of the herbicide, which is commonly a viscous liquid having a viscosity, for example, of 20-40 centistokes, has made it difficult for this requirement to be met.
In known proposals for promoting the creation of fine droplets of uniform size, the fluid is supplied to a rapidly rotating disc from which the fluid is ejected by centrifugal force. The face of the atomising disc over which the fluid flows is formed with radial grooves terminating in radially extending points. Examples of such proposals can be found in British Patent Specification Nos. 1515511 and 2008439. The radial grooves constitute channels along which fluid flows under the action of centrifugal force when the disc is rotated. At the radially outer ends of the grooves, the individual streams of fluid are ejected from the points and break up into fine droplets. When using such discs, however, it is not possible to control the spraying width or the droplet size to suit prevailing conditions.